Abstract. During the DOMINO (Diel Oxidant Mechanism
In relation to Nitrogen Oxides) campaign in
southwest Spain we measured simultaneously all quantities necessary to
calculate a photostationary state for HONO in the gas phase. These
quantities comprise the concentrations of OH, NO, and HONO and the
photolysis frequency of NO2, j(NO2) as a proxy for j(HONO). This
allowed us to calculate values of the unknown HONO daytime source. This
unknown HONO source, normalized by NO2 mixing ratios and expressed as a
conversion frequency (% h−1), showed a clear dependence on
j(NO2) with values up to 43% h−1 at noon. We compared our
unknown HONO source with values calculated from the measured field data for
two recently proposed processes, the light-induced NO2 conversion on
soot surfaces and the reaction of electronically excited NO2* with
water vapour, with the result that these two reactions normally contributed
less than 10% (<1% NO2 + soot + hν; and <10%
NO2* + H2O) to our unknown HONO daytime source. OH production from
HONO photolysis was found to be larger (by 20%) than the "classical"
OH formation from ozone photolysis (O(1D)) integrated over the day.